NYACK — After 10 years and $384.6 million, the New York State Thruway Authority is slowing the spigot of money that has held the Tappan Zee Bridge together.

The spigot's last drip will come in September, with completion of the final phase of a capital program that has replaced 98 percent of the TZB's original, and failing, deck. The project breathed new life into the 57-year-old span, enough, at least, to keep it safe and sound until a new $3.1 billion bridge rises alongside it.

"Structurally, the old bridge is in good functional condition, and we don't anticipate any additional construction projects before the new bridge is placed in service," said Ted Nadratowski, the Thruway Authority's interim director of maintenance and operations.

In fact, independent engineers gave the TZB a rating of 5 on a scale of 1 to 7 following its biennial inspection last year. The federally mandated process took seven months of hands-on examination and cost the Thruway Authority $550,000. Only new or young bridges tend to garner higher ratings.

Until five years ago, the TZB was mired in the 4s, a factor in the decision to replace more and more of the 3.1-mile-long deck and vanquish the notorious "punch-throughs" that unnerved drivers with sudden river views.

Nadratowski, who used to have day-to-day responsibility for the TZB and the Thruway between New York City and New Paltz, believes the rating will hold into 2017, when traffic is transferred to the first span of the new bridge temporarily. The old bridge's demolition will follow so that construction of the second span, scheduled to open in 2018, can be completed.

"All the routine maintenance tasks that we perform now will continue — on the deck, electrical system, substructure, moveable barrier — as well as the patrols, security, inspections," said Nadratowski.

The TZB has a dedicated maintenance staff of 57 men and women who work around the clock and the calendar. They claim about one-third of the New York division's $47.3 million operating budget in salaries and materials.

Care of the 2 percent of the deck that has not been replaced — the section leading into the toll plaza — will fall to these crews. Nadratowski said they may be spared some of the never-ending painting as the day of decommissioning nears.

The staff will also monitor the sensors that have been installed on the old bridge to detect any movement or vibration from the new bridge's construction.

"We don't have any reason to think there will be issues," said Nadratowki. "It's just good practice since we are so close, and since the causeway (the 2-mile section on the Rockland side) is supported on wooden friction piles rather than founded on bedrock."

At some point, the crews will begin training for the maintenance tasks they'll be expected to perform on the new bridge. They'll contend with dozens of bells and whistles that didn't exist 57 years ago, from LED lighting to smart technology for tracking structural stress.

"The new bridge will have plenty of maintenance needs; they'll just be different," said Nadratowski. "There's a lot of concrete, for example, and it's going to have to be sealed every five years, which means we'll be doing it continually."

What the Thruway Authority hasn't been able to do, however, is improve driving conditions for the 130,000 cars and trucks that cross the TZB every day and endure a higher-than-average risk of collision.

The old bridge still has steep grades that challenge the ability to maintain a constant speed and narrow travel lanes that redefine the dictum to "stay in lane." In the absence of shoulders, all it takes is one fender-bender to bring traffic to a standstill and back it up for miles.

"Everything we've done, or haven't been able to do, underscores the need for a new bridge," Nadratowski said. "It's not just a matter of the bridge being safe, but of driving conditions being safe — and driving conditions are unsafe."